The invention relates to a power miter saw for use in carpentry and other cutting crafts. More particularly, the invention relates to a pivot assembly for mounting a cutting unit for movement about an axis from a cutting position to a raised, non-cutting position to provide for zero axial movement or play of the cutting unit.
Power miter saws have been used as a quick and efficient way of making angular cuts in a work piece, usually a portion of a chair rail, base board, crown molding or aluminum siding. Typically, the power miter saw consists of a base having a turntable mounted thereon for rotational movement and a cutting unit mounted on the turntable for movement from a cutting position to a non-cutting position.
The turntable is usually metal and typically includes a slot through which the saw blade extends during cutting action. This slot is usually covered by a plastic or particle board cut-through kerf plate which, when the saw is first used, must be cut through to create a saw blade receiving slot in the turntable, referred to in the trade as a kerf. The purpose of the cut-through kerf plate is to provide support for the workpiece which is as close as possible to the saw blade so as to prevent the bottom of the workpiece from splintering and chipping during cutting. This phenomenon is commonly called bottom chip-out.
When cutting expensive plywoods of rare veneer or finished or costly hardwood lumber such chip-out can result in the finished cut piece becoming scrap resulting in excessive costs due to the material loss and the time the craftsmen must spend duplicating the scrapped piece. To provide a kerf in the kerf plate that is as close as possible to the sides of the blade, it is normal practice to assemble the miter saw, install the saw blade, turn the saw on and then lower the rotating blade through the uncut kerf plate to create a close tolerance kerf therein. The edges of the kerf are then as close as possible to the saw blade and provide excellent support to the workpiece when the saw is new. Unfortunately, axial wear which occurs during use of the saw allows the saw blade to shift axially which cuts the kerf wider. This soon destroys the close tolerance kerf and creates chip-out problems for the craftsman as will now be explained.
Additionally, the play created by the axial wear of the pivot assembly will compromise the axial precision of the saw blade as the cutting unit moves from the non-cutting position to the cutting position and through the workpiece. The increased play will result in a diminished accuracy of the cut in the workpiece.
The usual cutting unit pivot assembly is some form of a hinge which has a single boss or a pair of bosses mounted on the base in spaced relation, each of the bosses having a bore extending longitudinally along a common axis. The cutting unit typically includes a mounting member having a projecting tongue or shank which also has a bore extending longitudinally therethrough. The projecting shank is dimensioned to fit in the space between the bosses extending from the base, so that a pivot pin or shaft can fit through the aligned bores to connect the cutting unit mounting member and the base for pivotal movement relative to each other about a generally horizontal axis defined by the pivot pin.
In order to make controlled and precise miter cuts, and to prevent the saw blade from moving axially and enlarging the width of the kerf in the kerf plate, it is generally desirable that the hinge provide for only precise vertical movement of the cutting unit. That is, in moving from the non-cutting position to the cutting position, and vice versa, the cutting unit should not experience any axial movement relative to the base. In order to achieve this result, it has been necessary to manufacture the components of the hinge using parts which are first cast and then machined. This two-step process is expensive and is even made more so because of the exact tolerances which must be adhered to in machining the castings.
If exact tolerances are not maintained, there will be axial play in the hinge resulting in the saw blade engaging the kerf plate at varying axial positions. The axial inconsistency of the cutting action will cause accelerated wear in the width of the kerf in the kerf plate and result in loss of the close tolerance kerf necessary to prevent chip-out. Frequent replacement of the kerf plate will not solve this problem because axial play will destroy the close tolerance kerf in the new kerf plate.
Furthermore even if exact tolerances are maintained during manufacture, conventional hinge arrangements will over time, develop chip-out problems. Regular use of the saw causes wear in the hinge apparatus. Over a period of time, this wear will reduce the axial precision of the hinge assembly allowing the blade to shift axially, and the necessary close tolerance fit between the kerf and blade will be lost resulting in chip-out occurring. Prior miter saw designs do not solve the problem of chip-out resulting from wear induced axial play that widens the kerf in the kerf plate.
In summary, the problem that exists is how to provide a miter saw cutting unit hinge assembly which can be manufactured using relatively inexpensive and easily made parts, for example casting used in an as-cast, non-machined condition, and which has the ability to accommodate normal axial wear of the hinge components so that the precise axial position of the saw blade is maintained throughout the normal operating life of the miter saw to minimize widening of the kerf in the kerf plate.
Attention is directed to the following U.S. patents:
U.S. Pat. No. 4,934,233, Brundage et al., Jun. 19, 1990 PA1 U.S. Pat. No. 4,774,866, Dehari et al., Oct. 4, 1988 PA1 U.S. Pat. No. 4,581,966, Kaiser et al., Apr. 15, 1986 PA1 U.S. Pat. No. 3,706,332, George, Dec. 19, 1972
These patents generally show the state of the art in miter or chop saw hinge mounting assemblies. The references generally show that it is known in the art to mount the cutting unit using a hinge which allows for rotation of the cutting unit about a horizontal axis defined by a pivot pin for movement between a cutting and non-cutting position. However, the cited patents do not teach or disclose an apparatus using inexpensive and easily manufactured components and which has the added capability of being able to accommodate axial wear of the components of the hinge to substantially prevent axial deviation of the saw blade from the kerf made in the kerf plate when the miter saw was new.